1. Field of the Invention
This invention relates to the transmission of data signals over power lines, and more particularly to a passive signal coupler for transferring data signals between different power distribution systems.
2. Description of the Prior Art
The need for new, more effective control systems for building operations has never been greater. Now more than ever, businesses are turning to advanced central control technology to perform a number of tasks necessary for day-to-day operation. Until recently, however, the complexity and expense of system installation have prevented many building owners and managers from taking advantage of available technology. In many older buildings, it has proven too expensive or even impossible to install these systems because of hard-to-trace wiring or building construction obstacles. Even in newer buildings the cost of running hard control wiring to multiple remote points can be prohibitive. In either case, the costs associated with running wire are the most significant part of the cost of installation, in some buildings as much as 75 percent of the total cost.
Power line carrier systems effectively eliminate the need for major wiring jobs associated with control system installation. It utilizes the existing in-building AC power service to take the place of hard control wiring. The power line carrier systems can be used in a wide variety of applications, including environmental and energy management control, building and factory automation, remote lighting and process control. There are additional advantages for those who wish to employ control systems in temporary or leased locations. In these cases, the system can be moved easily to a new location or building, eliminating the need to rewire with every move or to leave the control wiring behind.
There are many advantages to using the power line carrier systems instead of hard wiring: installation becomes a much easier task; walls and ceilings are left intact; building occupants and tenants are not inconvenienced by messy installation and repair jobs; total installation costs are easier to estimate and control; and the control equipment may be moved about without rewiring.
Power line carrier systems are communication systems geared to pass commands from a master controller, typically a digital computer to energy-using devices. It is desirable to use the existing AC power lines for transmitting these commands between devices by using a radio frequency signal carrier. When large buildings or building complexes are involved, installation of the power line carrier system usually costs much less than installing new wiring between separate energy-using devices and the master controller.
It is likely that the transceivers that transmit and receive data signals are coupled to different power distribution systems. Also, the power systems may supply different voltages. Usually one power system is transformer-coupled to the other system. This is fine for providing power to operate equipment; however, the transformer attenuates the radio frequency signals required for the data signals transmission. Accordingly, the prior art has provided passive signal couplers to allow the data signals to bypass the transformers. These passive signal couplers couple the respective phases of the two power systems by means of a capacitor or a capacitor in series with an inductor.
This prior art solution, however, presents a serious safety hazard. In the event that a capacitor shorts out, then the two power distribution systems which may be supplying different voltages will be directly connected and may result in serious equipment damage and possibly fire. It may be that the fuse breakers will prevent damage. However, now there is the problem of having a building without power and locating the source of the power failure before the building may again be used for its normal business function.